The propagation of inverted Neel wall sections in a serial access memory system was proposed by L. J. Schwee in the publication "Proposal On Cross-tie Wall and Bloch-line Propagation In Thin Magnetic Films" IEEE Transactions on Magnetics, MAG 8, No. 3, pages 405-407, September 1972. Such a memory system utilizes a ferromagnetic film of approximately 81% Ni-19% Fe approximately 350 Angstroms (A) thick in which cross-tie walls can be changed to Neel walls and Neel walls can be changed to cross-tie walls by applying appropriate fields. Associated with the cross-tie wall is a section of inverted Neel wall that is bounded by a cross-tie on one end and a Bloch-line on the other end.
In such a cross-tie wall memory system, information is entered at one end of the serial access memory system by the generation of an inverted Neel wall section, formed by a cross-tie on one side and a Bloch-line on the other, that is representative of a stored binary 1 or of a non-inverted Neel wall section (i.e., the absence of a cross-tie and Bloch-line pair) that is representative of a stored binary 0. Such information is moved or propagated along the cross-tie wall by the successive generation (and then the selective annihilation) of inverted Neel wall sections at successive memory cells along the cross-tie wall. In the D. S. Lo, et al, U.S. Pat. No. 3,906,466 there is disclosed a propagation circuit for the transfer of inverted Neel wall sections at successive memory cells along the cross-tie wall. In the L. J. Schwee U.S. Pat. No. 3,868,660 and in the publication "Cross-tie Memory Simplified by the Use of Serrated Strips," L. J. Schwee, et al, AIP Conference Proceedings, No. 29, 21st Annual Conference on Magnetism and Magnetic Materials, 1975, published April 1976, pages 624-625, and in the publication "Cross-Tie/Bloch-Line Detection," G. J. Cosimini, et al, AIP Conference Proceedings, No. 3, 23rd Annual Conference on Magnetism and Magnetic Materials, 1978, published March, 1978, pages 1828-1830, there have been published more recent results of the further development of cross-tie wall memory systems.
In prior art cross-tie wall memory systems, the magnetic film that functions as the storage medium has the property of uniaxial anisotropy provided by its easy axis induced magnetic fields, which easy axis is generated in the magnetic film during its formation in the vapor deposition process. This easy axis provides a magnetic field induced anisotropy which constrains the generation of the cross-tie wall along and parallel to the easy axis. In the above L. J. Schwee, et al, AIP publication there are proposed serrated strips of Permalloy film, about 350 Angstroms (A) in thickness and 10 microns (.mu.m) in width, which serrated strips are etched from a planar layer of the magnetic material so that the strips are aligned along the easy axis of the film. After an external magnetic field is applied normal to the strip length, i.e., transverse the easy axis of the film, the magnetization along the opposing serrated edges rotates back to the nearest direction that is parallel to the edge. This generates two large domains that are separated by a Neel or cross-tie wall that is formed along the centerline of the strip. Cross-ties are energetically more stable at the necks of the serrated edges while Bloch-lines are energetically more stable in the potential wells between adjacent necks.
This serrated strip configuration, because of the contour of the opposing edges of the strip, provides the means whereby the cross-tie, Bloch-line pairs are structured at predetermined memory sections along the strip. However, because prior art strips have field induced uniaxial anisotropy imparted during deposition, such strips cannot be utilized to permit the use of nonlinear, i.e., curved, data tracks, which curved data tracks are essential to the configuration of cross-tie wall memory systems of large capacity or of digital logic function capabilities. In the L. H. Johnson, et al, U.S. Pat. No. 4,075,612 there is disclosed a design of the edge contour of a film strip of, e.g., Permalloy film of approximately 350 A in thickness and approximately 10 .mu.m in width. The edge contours are mirror images, one of the other, of asymmetrical, repetitive patterns of rounded edge portions. The edge contour of each opposing pair of rounded edge portions is substantially in alignment with the natural contour of the magnetization that is oriented around a Bloch-line, which Bloch-line is positioned along the cross-tie wall that is oriented along the geometric centerline of the film strip. The neck or narrowest point of the edge contour between adjacent rounded edge portions functions to structure the static or rest position of the associated cross-tie of the cross-tie, Bloch-line pair.
In the M. C. Paul, et al, U.S. Pat. No. 4,130,888 there is disclosed a cross-tie wall memory system and in particular a data track therefore that is formed of a strip of magnetic material having substantially zero magnetic field induced anisotropy. The data-track-defining-strip of isotropic material utilizes its shape, i.e., its edge contour induced, anisotropy to constrain the cross-tie wall within the planar contour and along the centerline of the film strip. Accordingly, the cross-tie wall is constrained to follow the path defined by the magnetic film strip which path may be configured into a major loop, or circular data track, configuration for large capacity memory storage.
In the E. J. Torok U.S. Pat. Nos. 4,030,591 and 4,075,613 there is utilized the data-track-defining-strip of isotropic magnetic film of the hereinabove referenced M. C. Paul, et al, patent to form a replicator of and a logic gate for cross-tie, Bloch-line pairs. The replicator is utilized as a magnetic switch or gate to selectively transfer cross-tie, Bloch-line pairs between merging, overlapping data tracks. This permits the configuration of a plurality of continuous data tracks into a major-loop, minor-loop configuration for a large capacity memory system. The logic gate is utilized as a magnetic switch to selectively perform the logic OR function or the logic AND function upon two merging, overlapping data tracks.
The use of the major-loop, minor-loop configuration permits the information bearing cross-tie, Bloch-line pairs to be circulated past a detector in the major loop, which detector determines the presence or absence of the cross-tie, Bloch-line pairs as indicative of the storage of binary 1's or 0's. This readout is nondestructive because the stored information is generally continuously circulated around the minor loops, but is transferred, or replicated, into the major loop for readout. This, of course, generates a greater than desirable latency time such as is common to all non-random access memory devices such as charge-coupled-devices, magnetic drums, disks, etc. In the present invention this latency time is substantially reduced while still providing nondestructive readout of the stored data word.